Digital color video cameras which create a digital video signal consisting of a brightness signal component and a color signal component are known in the art. The two signal components are immediately transformed into an analog color video signal. The analog video signal is suitable for the conventional transmission of video signals and for the conventional display of an image sequence on an ordinary TV monitor.
A modern digital color video camera based on so called CCD technology in its brightness range has a typical resolution of 8, 10 or 12 bits. This means it is capable of recording 255, 1023 or 4095 levels of brightness. In contrast, the brightness resolution of the human eye is very limited. The difference in brightness of two neighboring picture elements has to be approximately 1 to 2 percent to be recognized. A brightness resolution of more than 7 bits (which corresponds to 127 levels of brightness) does not produce a visible quality improvement when displaying images.
At the same time, pictures rarely show regions with a brightness value of zero and/or a maximum value. In other words, often there are no purely black and/or white regions in an image.
Submitting single images to a digital image processing procedure raises the contrast in such pictures. In order to achieve this, the generally analog color video signal has to be digitized first. Then, a maximum and a minimum brightness value are calculated in each image. An offset value and a gain value are determined therefrom. Then, the image data are modified by subtracting the offset value therefrom and by multiplying them with the gain value such that they span all the possible levels of brightness as fully as possible.
This known digital image processing existing in software does not—with a reasonable expenditure for hardware—increase the contrast in a sequence of images acquired with a digital color video camera to display them in real-time on a color monitor. However this is often demanded, for example, in real-time surveillance of objects or during the observation of endoscopic procedures during surgery.
A method of increasing the contrast in a sequence of images acquired with a black-and-white video camera is known. An analog video signal is processed with a hardware circuit at the camera output, and it is put out as a processed analog video signal. A hardware solution like this is offered by ADIMEC. The known process is not suitable for analog color video signals, and it can therefore not be used with a color video camera.